1. Field of the Invention
The present invention relates in general to cleaning a flue gas and in particular to a new and useful method and system for cleaning flue gas by regenerable magnesium dry scrubbing.
2. Description of the Related Art
In the power plant field, it is known to use a regenerable magnesium oxide process in either a dry or wet scrubber in order to remove contaminants from a flue gas. In this process, it is known that flyash and chlorides are contaminates that prevent the economic recycle of the magnesium values for reuse. It is well known to remove flyash prior to contaminant scrubbing and chlorides can be removed with a purge liquid.
U.S. Pat. No. 4,246,245 to Abrams, et al. discloses a side stream containing magnesium sulfite-bisulfite which is oxidized to sulfate and magnesium sulfate which is converted to magnesium hydroxide. This reference describes the use of a type S dolomitic lime which contains about 55% calcium hydroxide, 40% magnesium hydroxide, 2% magnesium oxide, and 0.2% water. This reference does not disclose oxidation inhibition, but rather, the product (magnesium sulfite) is oxidized to produce magnesium sulfate which, in turn, is returned to a reaction zone.
U.S. Pat. No. 4,388,238 to Abrams, et al. describes how sulfur dioxide is contacted with a magnesium sulfite solution in a scrubbing zone at a pH in the range of about 6 to 7.5 in order to produce magnesium bisulfite. It also teaches providing a minor regeneration stream which is subjected to oxidation to oxidize bisulfite and sulfite to sulfate. This reference also teaches the use of a type S hydrated dolomitic lime for the spray dryer at a weight percent of 3-15% suspended solids and at a stoichiometric ratio of 1.1 to 1.5 and injected at a temperature of 145.degree. to 165.degree. F. The product is calcium sulfite and sulfate, magnesium sulfite and sulfate, unreacted lime, and flyash. However, the presence of calcium sulfite and sulfate and magnesium sulfate is undesirable.
U.S. Pat. No. 4,623,523 to Abrams, et al. also teaches the use of hydrated dolomitic lime. This reference discloses the use of a confined zone within a flue gas stream to reduce sulfur dioxide by 10-90%. This reference does not provide for operating at SO.sub.2 removal rate higher than 90% and does not teach a method which uses a regenerable reagent, produces a salable product, and produces a small waste stream.
U.S. Pat. No. 4,472,364 to Bauerle, et al. teaches the use of dry scrubbing magnesium oxide. This reference does not teach the use of oxidation inhibition, chloride purge, magnesium sulfite internal recycle to a dry scrubber, and a slipstream treatment for chloride and sulfate removal. This reference teaches the production of a waste product and oxidation of the sulfite, not a suppression of oxidation.